This invention relates to power hammers used for driving piles, for breaking rocks and pavement, and for similar purposes.
Known power hammers of this type typically include a vertical housing having a cylinder in which a heavy ram or piston is arranged for vertical, reciprocating movement, and a relatively heavy anvil is carried within the lower portion of the housing so that it will be struck by the full force of the ram during its downward movement, and this impact force is transmitted by the anvil to the object being driven or crushed, usually through intermediate components such as a striker block and a drive cap when the object to be driven is a pile.
A combustion chamber is formed within the cylindrical housing between the lower surface of the ram and the upper surface of the anvil, and fuel and air are introduced into the combustion chamber by appropriate valving. Once started, the downward movement of the ram causes compression and heating of the air within the combustion chamber so that the fuel is ignited, and the force of combustion causes the ram to be pushed upwardly in the housing to its uppermost position, whereupon gravity overcomes such force and then the ram again moves downwardly to strike the anvil, and this cycle is repeated continuously.
To reduce the stroke of the ram and, therefore, the length of a power hammer, and to increase the striking rate of the hammer, it is now common practice to also provide the housing of the hammer with an associated compression or "bounce" chamber which communicates with the housing above the ram so that during the aforesaid upward movement of the ram air is forced into the bounce chamber and compressed, and this compressed air is then utilized to combine with the weight of the ram to drive it downwardly against the anvil.
Typical examples of conventional power hammers of the foregoing type are described in greater detail in U.S. Pat. Nos. 3,437,157 and 4,020,804.
In this conventional type of power hammers, the upward movement of the ram within the housing, and the attendant compression of air above the ram and within the bounce chamber, creates an upwardly directed force against the top of the housing which tends to lift the hammer off of the pile being driven. This upwardly directed force is resisted only by the overall weight of the hammer, and it is particularly significant to note that the weight of the anvil, which is relatively heavy (e.g. over 2,000 pounds in larger hammers), is not utilized in opposing such upward force because it must be arranged within the cylindrical housing for relative sliding movement with respect thereto to avoid transmitting destructive impact forces directly to the housing when the anvil is struck by the ram. Thus, the anvil generally rests upon the striker block, which is situated on the pile through a drive cap attachment, and the anvil, in effect, simply floats within the cylindrical housing during upward movement of the ram so that the weight of the anvil is not added to the weight of the hammer as a whole to oppose the upwardly directed forces generated by the ram being driven upwardly by the combustion forces in the combustion chamber. Accordingly, if such upward force is of a magnitude that may lift the hammer off of the object being driven or worked upon, the generally accepted solution to this problem has been to substantially increase the weight of the power hammer housing, which has the disadvantage of correspondingly increasing the cost of the equipment as well as rendering the equipment more difficult to handle and transport.
Additionally, most power hammers include an anvil retainer that is connected to the housing by bolts and that is provided with some projecting flange or the like which will engage the anvil and retain it in the housing when the anvil reaches its lowermost position in the housing. In many operations the relative movement of the anvil within the housing is small and the anvil seldom reaches its lowermost position, but in some operations, such as when a pile is driven into soft earth, the anvil will strike the anvil retainer and impose shock loads on the bolts which connect the anvil retainer to the housing. To alleviate this impact transmission problem, stacked disc washers have been placed between the anvil retaining portion of the housing and the heads of the bolts which connect the anvil retainer to the housing to absorb some of the shock load imposed on the anvil retainer. However, such mechanical shock absorbers are subject to wear in time, and increase the maintenance problems and costs associated with the equipment.
In accordance the present invention, the foregoing drawbacks of known power hammers are overcome by a unique arrangement through which the weight of the anvil is utilized to oppose the upwardly directed forces created by the ascending ram, while also providing an air cushion to absorb shock forces that might otherwise be imposed on the anvil retainer when the anvil is struck by the ram.